The present invention relates to an exhaust gas filter suitable for removing black smoke contained in exhaust gas from a diesel engine mounted on a bus, a truck, a forklift truck and the like. The present invention also relates to a method of producing such an exhaust gas filter, and an exhaust gas purification apparatus provided with such an exhaust gas filter.
Recently, in large cities, air pollution due to NOx and floating particulate matter has been more and more advanced, and has now become a serious social problem. It is announced that particulates in the black smoke discharged from the diesel engine occupy 30% to 40% of the floating particulate matter in the atmosphere, and mutagenic and carcinogenic components, e.g. polynuclear aromatic hydrocarbons such as benzpyrene are contained in the black smoke particulates.
An exhaust gas purification apparatus of the type which arrests or traps the particulates in an exhaust system, and then effects self regeneration is expected to be most effective for dealing with black smoke. This exhaust gas purification apparatus is provided in the exhaust system, and mainly comprises an exhaust gas filter for trapping the black smoke particulates in the exhaust gas, and a regenerator for burning to remove the trapped black smoke particulates. The regeneration in this regenerator may be effected by an electric heater, a burner, hot air, or a backwash, each of which has advantages and disadvantages.
The mainstream of such exhaust gas filters has heretofore been a ceramic monolithic wall flow type filter (hereinafter referred to as "ceramic monolithic filter") for trapping black smoke particulates disclosed, for example, in U.S. Pat. No. 4,364,761.
In FIG. 3 showing a longitudinal sectional view of a conventional ceramic monolithic filter, a reference numeral 11 denotes a cylindrical filter body made of ceramics, and a number of cells 12 and 13 are formed in this body 11, each extending axially. A plug 14 made of ceramic is provided in an exhaust gas inlet side end of each of the cells 12 and the plug 14 is also provided in an exhaust gas outlet side end of each of the cells 13. These cells 12 and 13 are so arranged that they alternate with each other.
In the exhaust gas filter of this construction, the exhaust gas containing black smoke flows through the cells 13 each having the closed outlet side end into the adjacent cells 12 each having the closed inlet side end via porous partition walls 17. The black smoke is filtered when it passes through the porous partition walls 17, and therefore the black smoke particulates are trapped by the porous partition walls 17. When the amount of the trapped black smoke particulates increases, the porous partition walls 17 are clogged by the black smoke particulates, so that a back pressure of an exhaust device of an associated diesel engine increases. Therefore, it is necessary to remove the trapped black smoke particulates from the filter when their amount exceeds a predetermined level, so as to suppress the increase of the load on the engine due to the increment of the back pressure.
The black smoke particulates contain a fixed carbon component and a soluble organic component soluble in an organic solvent. These components are both combustible, and burn out at 650.degree. C. or higher though this temperature slightly differs depending on the kind of engine and the load condition. Therefore, it has been proposed to burn out the trapped black smoke particulates by means of heating means, such as an electric heater, a burner and hot air, so as to regenerate the porous partition walls 17.
In order to decrease a thermal stress produced by a temperature gradient developing at the time of this regeneration combustion, the conventional exhaust filter element is made of cordierite which is a typical example of ceramic material of a low thermal expansion. For example, Japanese Patent Unexamined Publication No. 62-225249 discloses a cordierite honeycomb-structure catalyst carrier. It comprises a honeycomb structure which consists, by weight, mainly of 42% to 56% SiO.sub.2, 30% to 45% Al.sub.2 O.sub.3 and 12% to 16% MgO, and has a crystal phase composed mainly of cordierite, and whose porosity is not more than 30%. A thermal expansion coefficient of the honeycomb structure in its axial direction at a temperature of 40.degree. C. to 800.degree. C. is not more than 0.8.times.10.sup.-6, and the thermal expansion coefficient in a diametric direction at 40.degree. C. to 800.degree. C. is not more than 1.0.times.10.sup.-6. Japanese Patent Unexamined Publication No. 2-52015 discloses a porous ceramic honeycomb filter which consists, by weight, mainly of 42% to 56% SiO.sub.2, 30% to 45% Al.sub.2 O.sub.3 and 12% to 16% MgO, and has a crystal phase composed mainly of cordierite, and whose porosity is in the range of 40% and 55%, and a volume of pores of a diameter of not more than 2 .mu.m is not more than 0.015 cc/g.
As described above, the exhaust gas filter provided in the exhaust system of the diesel engine is regenerated after trapping the black smoke particulates in order to suppress the increase of the back pressure of the exhaust device of the diesel engine. In connection with the regeneration combustion, it is required that no crack develop in the surface and interior of the exhaust gas filter even if a thermal stress develops in the filter itself, and that no melting damage due to a temperature rise caused by abnormal combustion develop in the exhaust gas filter.
However, in either of the conventional examples, the cracks and the melting damage develop simultaneously in the exhaust gas filter due to an abrupt temperature rise and the high combustion temperature during the regeneration combustion of the exhaust gas filter.
The inventors of the present application actually conducted a test in which a conventional exhaust gas filter composed mainly of cordierite was mounted in an exhaust system of a diesel engine, and the black smoke particulates were trapped at 15 g per unit volume (1 liter) of the exhaust gas filter, and then the filter was regenerated by an electric heater. The thermocouple indicated that the temperature rise rate of a specified portion of the filter during the regeneration combustion reached 50.degree. C./min., and the combustion temperature reached 1,400.degree. C. Then, the exhaust gas filter 21 subjected to the regeneration combustion was removed from the exhaust system of the engine, and was inspected. As a result, it was confirmed that cracks 22 developed in the surface of the filter body as shown in FIG. 4, and that the interior of the filter body partially melted down.
Also, an exhaust gas filter which consisted, by weight, mainly of 42% to 56% SiO.sub.2, 30% to 45% Al.sub.2 O.sub.3 and 12% to 16% MgO, and had a crystal phase composed mainly of cordierite was mounted in an exhaust system of a diesel engine, and there was carried out a cyclic test in which after the engine was operated for 5 hours under predetermined conditions, the trapped black smoke particulates were burned out for regeneration. During the second operation, black smoke was discharged from an exhaust outlet port. Then, the exhaust gas filter was removed from the exhaust system of the engine and inspected. As a result it was found that large cracks were formed in the surface of the filter.
The conventional exhaust gas filter has a bulk specific gravity d of about 0.45 g/cm.sup.3. The bulk specific gravity of the exhaust gas filter can be calculated by dividing the weight of the exhaust gas filter by the volume of the exhaust gas filter. The volume of the exhaust gas filter is the product obtained by multiplying the section area and the height, and also includes the cells and the pores. A ceramic material, constituting a body of the exhaust gas filter used in the above test, was pounded into powder in an agate mortar to measure its specific heat h. Its value is about 0.204 cal/g.degree. C., and the product h.times.d of the bulk specific gravity and the specific heat is relatively small, i.e., about 0.092 cal/cm.sup.3 .degree. C.
Thus, in the conventional exhaust gas filter composed mainly of cordierite, the product h.times.d, the specific heat per unit volume, is relatively small, and therefore the filter is subjected to an abrupt temperature rise during the regeneration combustion effected after it traps the black smoke, and tends to reach a higher combustion temperature. From this, it is presumed that cracks and melting damage develop because of the abrupt temperature rise and high combustion temperature during the regeneration combustion.